Modulation of optical signals is commonly utilized to transfer information in modern communication systems. One method of modulation utilizes an electro-absorption modulator. One type of electro-absorption modulator includes a microwave waveguide electromagnetically coupled to an optical waveguide.
One type of optical waveguide includes a multiple-quantum well region formed of III-V compound semiconductors. The multiple quantum well region is produced by one or more quantum well layers of a low bandgap material interleaved between a corresponding number of barrier layers of a higher bandgap material. The bandgap energy of the quantum well layers is less than the bandgap energy of the barrier layers. Thus, a multiple quantum well structure composed of N quantum well layers has N+1 barrier layers.
Electrons and holes are confined in the same physical quantum well. As a result, the electrons and holes are in close proximity to each other and interact strongly to form exciton pairs. The exciton has a strong absorption similar to an atomic absorption. The electro-absorption modulator utilizes a microwave signal to change the excitonic absorption properties of the quantum well layers of the optical waveguide. The microwave signal is utilized to vary the electrical bias across the multiple quantum well layers. When biased at a first state, the external electric field across each quantum well layer results in a sharp optical absorption. When biased at a second state, the external electric field across each quantum well layer results in a lower optical absorption. Hence, the resulting modulated optical signal provided at an output of the electro-absorption modulator has an on-state and an off-state.
For optimal coupling between the microwave signal and the optical signal propagating in the electro-absorption modulator, the microwave signal should propagate along the device at the same velocity as the optical signal. However, the prior art devices are problematic in that the microwave and optical signals travel with different group velocities. Accordingly, the device efficiency and operating bandwidth is deleteriously reduced.